PS plates comprising a positive type sensitizing agent containing a diazo compound and a phenolic resin as main components or a negative type sensitizing agent containing an acrylic monomer or prepolymer as a main component have already been put into practical use as lithographic offset printing plate precursors. However, since all these printing plate precursors have a low sensitivity, these printing plate precursors are exposed to light with a film original, on which an image has been previously recorded, brought into close contact therewith to form printing plates. On the other hand, computer image processing techniques and large capacity data storage and communication techniques have recently enabled continuous computer operation including original input, correction, editing, layout and pagination. With this computer operation, an electronic editing system capable of instantly outputting data to terminal plotters via a high speed communications network or satellite communications network has also been put into practical use. In particular, this type of electronic editing system is in great demand in the field of newspaper printing requiring instant use. Furthermore, in the field of printing where a printing plate is reproduced as necessary based on an original stored in the form of film original, there is a tendency for originals to be stored as digital data in very large capacity recording media such as optical discs.
However, little or no direct type printing plate precursors designed to directly receive data from the output of terminal plotters to form a printing plate have been put into practical use. Even in stations where an electronic editing system is operated, data is outputted to a silver salt system photographic film. PS plates are then exposed to light with the silver salt system photographic film brought into contact therewith to form printing plates. One of the reasons for the above described conditions is that it is difficult to provide a direct type printing plate precursor having sufficient sensitivity to form a printing plate within a practical period of time using a light source in the output plotter (e.g., a He-Ne laser, a semiconductor laser).
An electrophotographic light-sensitive material can be a light-sensitive material with sufficient light sensitivity to provide a direct type printing plate.
Printing plate materials (printing plate precursors) utilizing electrophotography which are known include zinc oxide-resin dispersion system offset printing plate materials as described in JP-B-47-47610, 48-40002, 48-18325, 51-15766, and 51-25761 (the term "JP-B" as used herein means an "examined Japanese patent publication"). Such a printing plate material is designed to be subjected to an electrophotographic process which comprises the formation of toner images and then a treatment which comprises impregnation with a desensitizing solution (e.g., an acidic aqueous solution containing a ferrocyanide or ferricyanide) to desensitize the non-image portion. Offset printing plates thus treated have a printing capability of 5,000 to 10,000 sheets and thus are not suitable for printing more than 10,000 sheets. This system also is disadvantageous in that the desensitizing solution used includes a toxic cyanide compound.
Other known methods for the preparation of printing plates using electrophotography include methods which comprise the removal of a photoconductive layer from the non-image portion after the formation of the toner image. Examples of this type of electrophotographic printing plate precursors include those described in JP-B-37-17162, 38-6961, 38-7758, 41- 2426 and 46-39405, and JP-A-50-19509, 50-19510, 52-2437, 54-145538, 54-134632, 55-105254, 55-153948, 55-161250, 56-107246, 57-147656 and 57-161863 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In these methods, it is necessary to etch away the non-image portion from the electrophotographic photoreceptor to expose the hydrophilic surface. Therefore, is often a binding resin which is dissolved in or swells in an alkaline solvent is often used.
Examples of known binding resins which have been incorporated in electrophotographic printing plate precursors include styrene-maleic anhydride copolymers, vinyl acetate-maleic acid copolymers, vinyl acetate- maleic anhydride copolymers, and phenolic resins as described in JP-B-41-2426, 37-17162, and 38-6961, and JP-A-52-2437, 54-19803, 54-134632, 55-105254, 50-19509, and 50-19510. However, the use of such an electrophotographic printing plate precursor comprising an organic photoconducting compound has various problems. In particular, when a phenolic resin is used as the binding resin, the resulting electrophotographic printing plate precursor has a brittle film which is poor in printing resistance. Vinyl acetate-crotonic acid copolymers and vinyl acetate-maleic anhydride copolymers, are also insufficient in printing resistance. An electrophotographic printing plate precursor comprising an acrylic ester binder exhibits an excellent printing resistance, but unfortunately exhibits insufficient solubility in an alkaline aqueous solution which requires a long etching time. In the preparation of an electrophotographic printing plate using this system, since the light-sensitive layer the non-image area is etched with the developed toner image as resist layer, lateral etching called side etching easily takes place, causing a deterioration in picture quality.